This invention relates generally to melt-blown materials, and more particularly concerns a melt-blown material which has a fiber size gradient across its depth or Z direction.
Materials manufactured by melt-blowing are well known and widely used in a variety of ways in commercial, industrial, and household goods. The melt-blowing process is used to form webs of thermoplastic microfibers and involves heating a polymer resin to form a melt, extruding the melt through a die orifice in a die head, directing a stream of heated fluid, usually air, toward the melt exiting the die orifice to form filaments or fibers that are discontinuous and attenuated, and collecting the fibers on a drum or foraminous belt to form a web. Because the fibers are still tacky when they are collected, they bond together to form an integrated web.
The melt-blown process is well known and is described in various patents and publications, including NRL Report 4364, "Manufacture of Super-Fine Organic Fibers" by V. A. Wendt, E. L. Boon, and C. D. Fluharty; NRL Report 5265, "An improved Device for the Formation of Super-Fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young; and U.S. Pat. No. 3,849,241 issued Nov. 19, 1974, to Buntin, et al.
Melt-blown webs of microfibers are useful as filter media, absorbent materials, moisture barriers, insulators, wipes, and the like. Particularly, melt-blown materials appear to have a potential for use as filter media for HEPA and pre-HEPA filters.
A HEPA filter has a filtration efficiency of at least 99.97% to 0.3 micron particles. The efficiency of HEPA filters is measured in accordance with test procedures described in Military Standard 282, Test Method 102.1, using dioctylphthalate particles that average 0.3 micron at a face velocity of 10.4 to 10.5 feet per minute. The efficiency of the filter refers to the percentage of particles that are filtered out of the air stream by the HEPA filter. Filters are classified as HEPA only if they achieve the requisite 99.97% filtration efficiency. Filters having efficiencies from about 90 to 99.97% are referred to as pre-HEPA filters.
HEPA and pre-HEPA filters are used to filter air in clean rooms where integrated circuits and precision equipment are manufactured. HEPA and pre-HEPA filters are also used in filtering air for operating rooms to filter out bacteria and other contaminants which may be present in the air and harmful to patients.
Not only must HEPA and pre-HEPA filters provide the requisite filter efficiency, it is likewise important that the pressure drop across the filter be maintained as low as possible for a given filter efficiency. If the pressure drop becomes excessive across the HEPA or pre-HEPA filters, larger more powerful fans will be required to compensate for the excess pressure drop with the resulting increase in power and noise. Therefore, it is important that HEPA and pre-HEPA filters maintain the lowest possible pressure drop at a given efficiency rating over the useful life of the filter medium.
Typically HEPA and pre-HEPA filter media are produced from glass filaments which filaments range in size from 0.3 to 2.0 microns in diameters. Glass filter media are formed in sheets by a wet (papermaking) process. In order for glass fiber HEPA and pre-HEPA filters to perform at the requisite efficiencies, the filament sizes must be small to yield pore sizes within the filter that are sufficiently small to assure that the 0.3 micron particles do not pass through the filter media. Such filter media are usually formed as a single sheet with uniform distribution of glass fibers across the depth (Z direction).
Melt-blown filter media which approach pre-HEPA filter efficiency can be made by lowering the through-put of the melt-blown extruder from a conventional rate of about 4 to 5 pounds per inch of die width per hour (PIH) of polypropylene to about 1 PIH and increasing the amount of fluid used in attenuating and breaking up the polymer stream from a conventional rate of about 100 to 150 standard cubic feet per minute (SCFM) to about 250 to 325 SCFM. (Standard cubic feet per minute relates to a 20-inch wide die head. Therefore flow rates for cubic feet per minute per inch of die head width are calculated by dividing the SCFM value by 20.) The resulting melt-blown fibers have an average size of about 5 to 6 microns in diameters (ranging from about 0.5 microns to about 10 microns), about the same as regular melt-blown fibers, but the overall melt-blown web is more uniform than a conventional melt-blown web. While a conventional melt-blown web has a filter efficiency well below 70% at a pressure drop of about 0.1 inches of water, the improved melt-blown web, having similar sized fibers, has efficiencies from about 95% to about 99.26% at pressure drops from about 0.25 to about 0.36 inches of water.
Such a melt-blown filter medium can be further improved by cold calendering the melt-blown material at a pressure of from about 100 pounds per square inch to about 300 pounds per square inch. The resulting cold calendered melt-blown material when used as a filter medium has efficiencies from about 97.0% to about 99.57% with pressure drops from about 0.32 to about 0.65 inches of water.
Where a HEPA or pre-HEPA filter, comprising a single sheet of 0.3 to 2.0 micron glass fibers or of cold calendered, 0.5 to 10 micron melt-blown polypropylene fibers, is used to filter air containing a random distribution of particles ranging in size from very small, in the order of 0.3 microns, up to particles of much greater size, the single sheet glass fiber or melt-blown polypropylene fiber HEPA or pre-HEPA filter medium will tend to load up very rapidly with particulate matter as the large particles are trapped on the upstream surface of the filter. As a result, the upstream side of the filter rapidly becomes blocked with particulate matter, thereby increasing dramatically the pressure drop across the filter and shortening the filter's useful life.
In order to overcome such premature plugging and replacement, a depth filter medium may be used in place of a single sheet filter medium having uniform fiber distribution in its Z direction. Depth filters have layers of fibers which provide different filter efficiencies in the Z direction. The layers may be discrete laminated plies, they may have different densities, or they may result from mixing different fibers.
In the latter category, Till, et al, U.S. Pat. No. 3,073,735 discloses a process for making a depth filter material. The process includes a first fiber forming station for depositing fine plastic fibers having a diameter of from 0.5 microns to about 10 microns onto a collecting belt by means of a spray tube and air nozzle arrangement. At a second station, a fan blows staple length rayon fibers through a duct spaced from the spray tube and air nozzle in order to deposit rayon fibers having a diameter of 10 microns and greater on top of the fine plastic fibers deposited by the spray tube and air nozzle. The two fiber-depositing stations produce cone-shaped fiber patterns which overlap, thus producing a mixture of fibers in the center of the resulting web. The web has a gradient of fiber sizes from small thermoplastic fibers on one side to large staple fibers on the other side, and is said to function as a depth filter.
Pall U.S. Pat. No. 4,032,688 discloses a method for forming a melt-blown filter medium on a rotating mandrel. By tilting the die head with regard to the axis of the mandrel, the resulting web has a greater density of fibers near one surface and a lower density of fibers near the opposite surface. The fibers themselves, however, appear to be of uniform average diameter across the depth of the filter media.
As previously stated, depth filter media are also produced by laminating webs of material having different filter efficiencies or characteristics. For example, Cary U.S. Pat. No. 4,011,067 discloses a filter medium produced from either melt-blown or solution-blown microfibers. The filter medium is a plied medium having a base porous web, an intermediate layer of microfibers, and a top porous web. The outside layers contribute only a minor portion, normally less than 20% of the pressure drop, and are typically non-woven fibrous webs such as polyethylene terephthalate. The intermediate layer of microfibers is of sufficient thickness to produce a HEPA filter.
Wadsworth et al, U.S. Pat. No. 4,375,718 discloses a process for manufacturing an electrostatically charged filter medium. The fibers in the filter medium may be polypropylene and may be formed by melt-blowing techniques. The fiber sizes for the melt-blown fibers are disclosed to be from 0.3 to 5 microns in diameter. The melt-blown filter medium is then plied on either side with a contact web of non-woven cellulosic fibers such as cotton, rayon, wood pulp, or hemp, or mixtures of those fibers. The non-woven contact web has specific electrical properties which will accept an electrical charge.